Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations only declare the generic mutation profile (readOnly=false, destructive=false, idempotent=false); the description adds the material context: sandbox/test money, free to call, requires the user's account, returns an approval link rather than completing payment, and that credits are consumed after the monthly allowance and never expire.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Single dense paragraph, but front-loaded with the action and the price table, then the next-step instruction, then the caveats. Every clause carries information an agent would otherwise have to guess.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

No output schema exists, and the description compensates by stating what comes back (a PayPal approval link) and what the agent must do with it. Combined with pricing and sandbox status, nothing needed to invoke or sequence the tool is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100% and the enum already constrains 'pack', so the baseline is 3. The description goes beyond the schema by mapping each enum value to a credit amount and a price, which is decision-relevant meaning the schema does not carry.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('Start a one-off credit purchase') and enumerates exactly what can be bought with concrete prices per pack. An agent can distinguish it from confirm_credit_purchase, which is named explicitly as the follow-up step.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives the full workflow: call this, show the PayPal approval link to the user, then call confirm_credit_purchase with the order_id. It also names the alternative path (user approves on PayPal's page) and the constraint (sandbox only, never paste card details).

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.